The invention relates to a heat-hardenable binder mixture based on organic resins.
Many and varied chemical reactions have been proposed and also used, to harden, by crosslinking, the binders in paint films. The chemical bonds formed during the crosslinking reaction frequently do not satisfy all requirements which paint films have to meet. Thus, ester bonds are sensitive to hydrolysis, and amine groups can form hydrophilic imperfections in the stoved film.
In the case of 2-component systems, frequently one of the reactive groups must be blocked to prevent premature reaction. The compounds then evolved during stoving pollute the environment, in particular in the case of amines or phenols.
Electrocoating lacquering has become widely established in recent years for primer-coating electrically conductive substrates. Hitherto anodic electrocoating primer-coating has been prominent. The resin binders used for this purpose belong to the resins containing carboxyl groups, for example to maleate oils, maleated epoxide resins, alkyd resins, acrylic resins and, in particular, to maleated polybutadienes. These resins are rendered water-soluble by salt formation, chiefly with amines, and deposited by direct current at the anode in the electrocoating bath. However, the anodic electro-coating primer-coating process has serious disadvantages. Thus, electric deposition at the anode evolves oxygen, which can modify the resins depositing at the anode in a serious, unfavorable manner. Furthermore, iron ions from the anode enter the solution and are contained as imperfections in the stoved film and lead to discoloration and spots. They cause qualitative disadvantages in particular by salt formation and hence by lowering resistance to water and corrosion resistance.
The cathodic electrocoating primer-coating process developed in recent years to the marketable stage displaces in an increasing extent the anodic process, since the deficiencies described above are largely avoided. Thus, hydrogen, which does not affect the resin binder, forms during the deposition process at the cathode, where the paint film is now deposited.
The binders suitable for cathodic deposition predominantly contain amino groups, which are neutralized by means of acids to obtain solubility in water (German Offenlegungsschrift No. 2,345,044).
However, hydrophilic amino groups remaining in the stoved film are a disadvantage, since they cause lowering of the corrosion resistance. Although, in the crosslinking, the amino groups are converted by reaction with blocked isocyanates into the less hydrophilic urethane group, the blocking agents, such as, for example, phenol or ketoximes, are simultaneously liberated.
Depending on the type of structure of the amino groups necessarily contained in the resin, these groups can also be thermally eliminated by .beta.-elimination (German Offenlegungsschrift No. 2,363,074 corresponding to U.S. Pat. No. 3,935,074 and German Offenlegungsschrift No. 2,753,861). The resulting contamination of the exit air by large amounts of eliminated protective groups from the blocked isocyanates and amines must also be considered a disadvantage of this process.